Conventionally, in the printing and plate-making field and the medical field, solution waste generated along with the wet process for image forming materials has caused problems regarding workability, and in recent years, a decrease in the processing solution waste has been strongly demanded in terms of environmental protection and room saving. Thus, a technique for light heat photographic material for a technical photographic use is demanded in which exposure can be sufficiently carried out using a laser image setter or a laser imager, and sharp and bright images with high resolving power can be achieved. As such techniques, methods are well known which are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,487,075 and D. Morgan, l Dry Silver Photographic Materialsl (Handbook of Imaging Materials, Marcel Dekker, Inc. page 48, 1991), etc. These photosentive materials are referred to as thermally developable photosensitive materials comprising a support having thereon an organic silver salt, a photosensitive silver halide and a reducing agent.
Conventionally, these thermally developable photosensitive materials are characterized in that they are thermally developed at temperature of 80 to 140.degree. C. so as to obtain images without fixation, so that the silver halide and the organic silver salt in an unexposed portion are not removed and remain in the photosensitive materials.
Accordingly, the remaining silver halide and organic silver salt cause an increase of fog in the unexposed portion, staining the unexposed portion and discoloring an image tone of the developed silver into a warm black tone when storing the photosensitive materials for a long time. Specifically, in cases where an amount of silver is increased to obtain a sufficient image density available for a practical use, there exists a problem that diagnosis ability is lowered because of the increase of fog, so that the thermally developable photosensitive material with small amount of silver by which high light-sensitivity and high image density are attained has been desired.